Apparatus for frequency compression and expansion



March 24, 1959 c. s. REIS 2,879,337

APPARATUS FOR FREQUENCY COMPRESSION AND EXPANSION Filed Jan. 3, 1955 2Sheets-Sheet 2 E V 1\ l6 I l zo IQHQ-VP ZAs' Fs H iaWZ ie a e s s 4 a 2i o TRANSMISSION RE'CE/V'k Q III 5:: Q

1 v T IE i L .57 T9 m H31 T T 2 s v IIIII r-ro T m n m I 3 o: 2 TB :51113: 1

Q r, III

Q T's 111 3:11 T.

Q III T I? 0 T5 Q Tl? E III. r

Q I T Ill T? I; T8

Ii I T IE II5 2| III. T III. n; I I5. TB 111 r, 33-5 n INVENTOR.

.6 T, CHARLES SIZE/s BY 19 I9 T, y I

' ATTUPAJE'FS zofisnanv k 15 14 13 12? TIO 9 a a s 5 3 2 5 APPARATUS FORFREQUENCY COMPRESSION 5 AND EXPANSION Charles S. Reis, San Bruno, Calif.

Application January 3, 1955, Serial No. 479,619

3 Claims. (Cl. 179--15.55)

Generally, my invention relates to a method and apparatus for frequencycompression and expansion. More particularly, my invention relates to amethod and apparatus for frequency compression and expansion of speech.

In live, connected speech, the duration of basic speech elements(phonemes) exceeds the minimum required for perception by a listener.Analysis of the wave form of speech shows that the basic wave pattern ofan element will repeat. Thus, if a small sample is obtained whichincludes this redundant wave pattern and reproduced and repeated untilthe original time base is re stored, the element will be synthesized. Ithas been shown, statistically, that in average speech there areapproximately ten such elements per second (phonetic rate). Thus, ifrepresentative samples of the redunancy 3 are obtained at the rate often per second or more, transmitted or recorded, and then repeated torestore the original time base, intelligible speech is cognated.

In one particular experiment, the sampling frequency fulfilled the aboverequirements and a 20% sample was obtained. After reproduction andrepetition to restore the original time base, the reproduced speech wasintelligible. Thus, it is seen that by properly sampling, the durationof speech may be considerably reduced prior to transmission and thenrestored to its original time base at the receiving end. Consequently,more intelligence may be transmitted during a given amount of time,permitting greater utilization of existing communication systems.

Another way in which existing communication systems may be more fullyutilized is to reduce the channel band width required for thecommunication of the intelligence, thus permitting more channels for agiven frequency band. It has been estimated that a band of frequencies100 cycles wide with a signal-to-noise ratio of 20 db is sutficient fortransmission of telephone quality speech.

One method for time or frequency compression and expansion employsmagnetic tape apparatus. This apparatus is provided with a continuousloop tape and a revolving member having a plurality of magnetic headsspaced about its periphery. The relative velocity between the tape andthe heads and the spacing of the heads determine the sampling frequency,the frequency compression or expansion, and the interval of each sample.Playback apparatus converts the recordings back into intelligible speechby proper frequency expansion and gepetition of the samples to restorethe original time ase.

Magnetic tape apparatus has many disadvantages. For example, the motionof the tape and revolving heads must be accurately controlled in therecording and playback operations, and the two operations must beaccurately synchronized. The equipment is bulky, re- 70 quiring a largespace for its installation, expensive to construct, and requiresconstant maintenance. Further,

the amount of sampling is limited by the physical dimensions of theheads.

It is an object of my invention to provide a method and apparatus forfrequency compression and expanslon.

It is another object of my invention to provide a method and apparatusfor sampling a frequency spectrum.

It is another object of my invention to provide a method and apparatusfor sampling a frequency spectrum in which the sampling intervals areeasily controlled.

It is another object of my invention to provide a method and apparatusfor sampling a frequency spectrum in which the sampling frequency may beeasily controlled.

It is another object of my invention to provide a method and apparatusin which the compression and expansion of frequency may be readilycontrolled.

It is another object of my invention to provide a method and apparatusin which the compression and expansion are instantly cognated.

It is a further object of my invention to provide a method and apparatusin which the sampling and frequency compression and the frequencyexpansion and repetition are easily synchronized. v

It is still a further object of my invention to provide a method andapparatus of the above character which is inexpensive to construct, doesnot require constant maintenance, and which occupies a minimum of space,and which may be used in conjunction with existing communicationequipment.

Other objects and advantages of this invention will be apparent from thedescription thereof made in connection with the accompanying drawings.

Referring to the drawings:

Figure 1 shows a complete system for sampling and compressing, expandingand repeating the sampled frequency;

Figure 2 is a perspective view of my system; and

Figure 3 is a schematic illustration of the operation of my apparatus.

Generally, I convert the frequency spectrum into a compression andrarefaction wave train travelling in a liquid. The optical density ofthe liquid varies in accordance with the compression and rarefactionregions of the wave train. I then scan the liquid with a source ofelectromagnetic energy, such as a light beam. The amuont of energytransmitted transverse to the liquid is dependent upon the opticaldensity of the liquid. A transducing device receives the energytransmitted by the liquid and converts it into an electrical currentwhich has amplitude variations dependent upon the density and afrequency determined by the relative velocities of the light beam andthe wave train.

Referring to Figure l, the block 11 represents apparatus whichincorporates my invention and which samples and compresses the frequencyspectrum. Block 12 represents a transmitting system which may comprise atelephonic communication system, or a radio communication system. Block13 shows apparatus of my invention which incorporates means forexpanding the frequency spectrum and repeating the expanded frequencysamples.

The electrical frequency spectrum to be sampled is applied to the inputterminal 16 of the glass-sided storage cell 17. The cell is providedwith transparent walls 18 and 19 and is filled with a transparent liquid21. An electromechanical transducer 22 is inserted in one end wall ofthe cell and placed in contact with the liquid. The electricalexcitation of the transducer causes a dilation and contraction in thedirection of its thickness. These vibrations are transferred to theliquid and cause a series of compresison and rarefaction waves whichmove forward from the transducer at a velocity determined by theparticular liquid under consideration. Thus, the density of the liquidvaries in the direction of travel in a manner which corresponds to thefrequency. to be sampled. At the far end of the cell, absorbent material23 is provided to absorb the waves traveling down the liquid.Attenuation of the waves as they travel is negligible for the distancesemployed. The variations in density of the liquid provide a change inits transmission characteristics and index of refraction along itslength.

Means are provided on one side of the storage cell for sweeping arecurrent energy beam along the cell at a predetermined velocity andrecurrence rate. For example, the beam may be a short persistence spotformed on the screen of a cathode ray tube 26, having cathode element27, focusing elements 28, and deflection plates 29. A sweep generatorsupplies a deflection voltage which causes the electron beam to travelacross the fluorescent screen (see Figure 2). Thus, it is seen that thecathode ray tube provides a light source which can be made to sweepalong the storage cell at any desired velocity. By providing appropriatecontrolled (or synchronized) delay circuits in the sweep generator, therepetition rate of the sweep may be accurately controlled. Although Iprefer to use a cathode ray tube, it should be understood that othermeans such as revolving mirrors or revolving lenses may be employed toform a recurrent scanning source.

On the other side of the storage cell, a lens 31 intercepts thetransmitted energy and focuses the energy on the receiving device 32.This device generates an electrical current which varies in conformitywith the energy transmitted across the liquid at any given instant. Whena scanning light is employed, the receiver may be a photosensitivedevice, for example a photosensitive transistor, phototube or photocell.

The apparatus 13 is similar to the apparatus 11 and comprises a storagecell 36, a cathode ray tube 37, lens 38 and receiving device 39.

The apparatus 12, which is labeled transmission and reception may be acombination of transmitter and receiver, a tape recorder with playbackmeans, conventional telephone communication circuit, or any othersuitable type of communication system.

Operation of my apparatus is as follows: the intelligence to be sampledis converted into electrical current variations and supplied to the line16. Thus, if speech is to be sampled, it is converted into electricalcurrent variations by means of a microphone and suitable amplifiersamplify the voltage to provide a voltage suflicient to drive thecrystal. These electrical current variations are converted torarefaction and compression waves in the liquid which progress as a wavetrain in the direction indicated by arrow 41. The compression andrarefaction regions correspond to the frequency of electrical currentapplied to the crystal and have a density which correspond to the signalamplitude. The light beam is caused to scan the cell in the samedirection of propagation at a velocity which is less than the velocityof propagation of the waves along the liquid. Thus, there is a relativevelocity difference between the light beam and the waves travelling downthe storage cell.

In Figure 3, I have shown an input frequency 51 made up of elements 1through 18. Only the intervals of the elements are shown, but thesignals which they represent may have any frequency and amplitudecharacteristic. The waves 51 are shown at 52 as they appear in thestorage cell at various instances of time t -t The spot of light 53 isshown at the positions it will have at these instances of time assuminga scanning velocity equal to one-half of the velocity of the wave trainand scanning which is repetitious without delay between scans. Thevelocity vector V represents the direction and velocity of the wavetrain and vector V represents the velocity and direction of the scanningspot 53. The output of the receiver 32 is shown at 54. For the conditionassumed (V /zV,) the sampling which results is a 50% sampling, and thefrequency of the elements is divided by 2, as shown at 54. It is to benoted that by decreasing the relative velocity difference of the wavetrain and scanning beam, the frequency division is in creased, i.e.,greater frequency compression. If V, is the velocity of the wave trainand V the velocity of the scanning beam, V, may be expressed as someconstant times the velocity V, or

V,=kV,

The frequency which results for any relative velocity may then beexpressed as where f is the compressed frequency, f is the originalfrequency, and k is the constant in Equation 1. For'continuous,repetitive scanning, the percent sampling is also related to thesevelocities and may be expressed as:

Percent S=l00(1--k) Where scanning is not continuously repetitive butincludes delays or intervals, the percent sampling is dependent uponboth the relative velocity of the wave train and scanning light and thescanning frequency.

In the example assumed for purposes of illustration, the frequency isdivided by two and the time base remains the same; thus, a channelhaving one-half the frequency band is required for transmission of theoriginal frequency spectrum.

For expansion it is necessary to reconvert the sampled and compressedfrequency, the scanning velocity is made greater than the wave trainvelocity by scanning in an opposite direction. Referring again to Figure3, I have shown the output of the receiver 32 at 54. The output of thisreceiver is supplied to transmission means of the type previouslydescribed and then applied at the receiving end to a storage cellsimilar to the one previously described. V indicates the direction andvelocity of the wave train and V the direction and velocity of thescanning spot 56. Here again the various instants of time arerepresented by the schematic storage cells shown, and the relativeposition of the scanning beam is shown for the particular instances oftime chosen. The output of the transducer 39 is shown at 57. Scanning isopposite to the direction of travel of the wave train. In the particularinstance shown, the velocity V, is equal to the velocity V, and withthese velocities, the frequency of each segment is doubled, the timebase of each sampled element is divided by 2 (i.e., restored to theoriginal time base), and the samples are repeated. Thus, if the originalsampling was statistically correct, the resulting output will, ingeneral, give intelligence which corresponds quite accurately to theinput. For the frequency expansion, the frequency of the output wave isnow given by Synchronization between the compression and expansion maybe readily obtained by transmitting a synchronizing signal which willtrigger gate and time base controlled networks that control the scanningvelocities of the light beam.

Thus it is seen that I have provided a method and apparatus in which thesampling may be easily controlled and in which the relative velocity ofthe scanning beam and the wave train travelling down the liquiddetermine the amount of compression or expansion of frequency. Thesampling frequency and interval may be easily varied by controlled delayof the scanning beam or it may be varied by placing an aperture betweenthe light beam and storage cell so that only a small portion of the wavetrain travelling down the cell is sampled.

I claim:

1. In apparatus of the character described, a storage cell filled with aliquid connected to receive a frequency spectrum whereby a rarefactionand compression wave train corresponding thereto is formed in the cellliquid, an energy beam which scans the said cell, and a photosensitivetransistor for receiving the energy transmitted by said cell havingvariations in intensity which correspohd to the density of liquidscanned and converting the transmitted energy into electrical currentvariations corresponding to the variations in intensity.

2. In apparatus of the character described, a storage cell filled withliquid connected to receive a frequency spectrum whereby a rarefactionand compression wave train corresponding thereto is formed in theliquid, at light beam serving to scan the said cell in the direction ofthe wave train .formed on the screen of a cathode ray tube, lens meansfor focusing the energy transmitted by said cell, and receiving meansfor receiving the focused energy and converting the said energy intoelectrical current variations which correspond to the variations sionwave train corresponding thereto is formed in the liquid, a light beamadapted to scan the said cell formed by a cathode ray tube, a firstphoto-sensitive transistor for receiving the energy transmitted by saidcell and converting thefitransmitted energy into electrical currentvariations which correspond to the density of the liquid scanned by saidlight beam, transmission and reception means connected to receive saidcurrent variations, a second storage cell filled with liquid connectedto receive said transmitted electrical current variations whereby acompression and rarefaction wave train corresponding thereto is formedin said liquid, a second light beam adapted to scan the said cell formedby a cathode ray tube, and a ;p hotosensitive transistor for receivingthe energy transmitted by said cell and converting the same intoelectrical current variations which correspond to the original frequencyspectrum.

References Cited in the tile 0! this patent UNITED STATES PATENTS1,671,151 French et a1. May 29, 1928 2,312,835 Hansell Mar. 2, 19432,509,545 Walton May 30, 1950 2,513,520 Rosenthal July 4, 1950 2,623,942Schlesinger Dec. 30, 1952

